Polarizers are important components of liquid crystal displays. Liquid crystal displays (LCDs) are widely used components in applications such as, for example, Notebook Personal Computers (PCs), calculators, watches, liquid crystal color TVs, word processors, automotive instrument panels, anti-glare glasses and the like. Typically, polarizers are used in the form of film, the polarizer film. In an LCD, the liquid crystal elements are generally sandwiched between two layers of polarizing films (also referred to as polarizer film herein) which regulate the incident light that enters the liquid crystal producing an on-and-off contrast.
The polarizing film traditionally comprises a polymeric film, a colorant and other optional layers, collectively referred to as the polarizing film. The polymeric film is traditionally a stretched polymer film such as, for example, polyvinyl alcohol (PVA). The colorant is usually iodine or a dichroic dye that is absorbed on the polymer film. This arrangement may then be coated or sandwiched on both sides with a substrate such as, for example, polyethylene terephthalate (PET), polymethyl methacrylate (PMMA), triacetyl cellulose (TAG), and the like. This may further be coated with an adhesive layer, protective layer, and the like.
The nature and quality of the polymeric film influences the performance of the polarizing film. Traditional substrate film materials such as stretched PVA are increasingly found to be inadequate in performance. Their limitations have become apparent with increasingly sophisticated applications for polarizer films and LCDs. More and more, the environment for use of these materials is becoming increasingly harsher in terms of temperature, humidity and the like. PVA films lack the needed heat and humidity resistance, strength, dependability, ease of use and ease of processing. Furthermore, they frequently suffer from deterioration of optical properties, such as a decrease in polarizing efficiency when exposed to high humidity/heat environment.
Several attempts have been made to improve the performance of polarizer films with limited success. U.S. Pat. Nos. 5,310,509 and 5,340,504 disclose polarizing films based on water-soluble organic polymers such as polyvinyl alcohol and dichroic dyes. U.S. Pat. Nos. 4,824,882 and 5,059,356 disclose polyethylene terephthalate ("PET") films for polarizer applications. U.S. Pat. No. 5,318,856 discloses films of polyvinyl alcohol, polyvinyl formal, polyvinyl acetal and polyvinyl butyral. U.S. Pat. No. 4,842,781 disclose's films of polyvinyls, polyester and polyamides. These polymers, however, still have the same disadvantages of PVA, especially in thermal and humidity resistance.
U.S. Pat. No. 5,071,906 discloses a polarizing film comprising a uniaxially stretched PVA having a degree of polymerization of about 2,500-10,000, and a colorant. While this is a slight improvement over traditional lower molecular weight PVA, it still suffers from the disadvantages of PVA.
Past attempts to improve the overall performance of polarizer films involved increasing dye concentration or film thickness, but such exercises do not achieve the desired end result because of the following reasons.
The quality and utility of polarizers depend on properties such as the polarizing efficiency ("P.E." also referred to as extinction) and single piece transparency ("T.sub.sp ") of the dye-based polarizer film. Polarizing efficiency (P.E.) and single piece transparency (T.sub.sp) of dye-based polarizer films are defined by the following formulas: ##EQU1## where the transmissions (T.sub..perp., T.sub.//) of transverse and parallel polarization with respect to the draw direction are related to polarizers through the following formulas: EQU T.sub.1 =T.sub.0 .times.10.sup.-A.sbsp.1, i=.perp. or //, T.sub.0 is the Fresnel reflection factor. EQU A.sub.// =cd.times.(.epsilon..sub.// &lt;cos.sup.2 .theta.&gt;+.epsilon..sub.195 &lt;sin.sup.2 .theta.&gt;) EQU A.sub..perp. =1/2cd.times.(.epsilon..sub.// &lt;sin.sup.2 .theta.&gt;+.epsilon..sub..perp. (1+&lt;cos.sup.2 .theta.&gt;))
where
c is the concentration of dye in the film. PA1 d, the film thickness PA1 .theta., the angle between dye and film draw axes PA1 &lt; &gt; implies the orientation average and PA1 .epsilon..sub.1 components of the molecular absorptivity tensor of the dye, i.e., ##EQU2##
As the above formulas imply, the polarizing efficiency and the single-piece transmission are interrelated. Therefore, if one attempts to improve P.E. by increasing the dye concentration (c) or the film thickness (d), the transmissions rapidly decline producing a very dark polarizer.
To improve the overall performance of the polarizer, both the transmission and P.E. must be improved, This implies that the dyes that are used must have low transverse absorption (absorption of the light by the dye in the transverse direction to its molecular axis), dissolve in the polymeric film uniformly and develop high orientation when the polymeric film is oriented. It is difficult to achieve high P.E. when the dye is used with conventional polymers. When dyes are used with conventional semicrystalline polymers, the dyes tend to reside in the amorphous region. The order parameter of the amorphous region is significantly lower than the overall order parameter. If, however, one chooses to go with fully amorphous conventional polymers, such polymers may dissolve dyes more or less uniformly throughout the sample, but it is very difficult to develop a highly oriented structure and any so developed oriented structure is thermally unstable.
Liquid crystal polymers are known for their potential to achieve high degree of orientation. For example, one can achieve an orientation function (or order parameter) greater than 0.9 with liquid crystal polymer films. In contrast, the achievable order parameter for conventional polymers such as PVA is rarely greater than 0.8. The order parameter is defined as: ##EQU3## Since dichroic dyes typically have a rod-like molecular configuration with an aspect ratio of 3 or greater, if a such dye is uniformly blended with a suitable liquid crystal polymer that has high order parameter, then it is conceivable that without increasing the concentration of the dye, one may be able to achieve a high polarizing efficiency with good transmission.
In view of the foregoing and other advantages of liquid crystal polymers, it would be desirable to have polarizer films comprising liquid crystal polymers and dichroic dyes. Thus, if one can blend dichroic dyes uniformly with liquid crystal polymers in sufficient amounts and in such a manner that during orientation of the polymer the dye molecules also orient along with the polymer chains, this would result in a high degree of orientation of both the dye molecules and polymer chains (which can be measured by the dichroic ratio of the blend). For this reason, liquid crystal polymers would be ideal candidates for polarizer film applications. In fact, some attempts have been made in the past to use such polymers for polarizer applications, but they also have some major disadvantages.
Japanese patent application JP 62-28698 (filed Feb. 10, 1987) discloses a polarizing film consisting of a thermotropic liquid crystal polyester film with a dichroic coloring matter dyed and oriented, wherein the polymer is a copolyester of a hydroquinone derivative (A), a terephthalic acid ingredient (B), an isophthalic acid ingredient (C) and a parahydroxybenzoic acid ingredient (D), with the molar ratio of A to D being in the range 5:95 to 70:30% and the molar ratio of B to C being in the range 50:50 to 100:0%. The disclosed polymer compositions are difficult or nearly impossible to make. Additionally, the monomer ratios disclosed for those polymers do not necessarily yield a balanced formula for preparing liquid crystalline polymeric compositions. Moreover, if even one could make such polymers, any films from such polymers are likely to be substantially deficient in optical transparency, which therefore would limit and/or prevent any potential utility as polarizing films, especially in stringent environments.
U.S. Pat. No. 4,840,640 discloses the use of "liquid crystalline polyethylene terephthalate-parahydroxybenzoic acid," formed by copolymerizing a polyethylene terephthalate component (A) with a parahydroxybenzoic acid component (B) with the A:B molar ratio being in the range 40:60 to 5:95. Optical properties, especially light transmittance are a concern with such compositions. Additionally, such compositions have to be first blended with a dichroic acid and then formed into a film through a die at a high shear rate to achieve satisfactory film orientation and light transmittance. This not only increases the processing steps, but also yields films with inadequate performance.
Accordingly, it is an object of this invention to provide a polarizing film which has high extinction (high P.E.) useful for polarizer applications and liquid crystal display devices.
It is another object of this invention to provide high extinction polarizers which also have good transmission in the desired wavelength light and high dichroic ratio.
It is an additional object of this invention to provide liquid crystal polymer compositions that can be blended with suitable dichroic dyes and then formed into films useful for polarizer applications.
It is yet another object of this invention to provide liquid crystalline polymers which can be blended with dyes and formed into films with high orientation, optical transparency, moisture resistance and heat resistance with minimal processing needs.
Other objects and advantages of the present invention shall become apparent from the accompanying description and examples.